Decomposing an object converts it into simpler objects. If you decompose a filled fractal, for example, it will become a filled polygon. If you decompose that once more, it will become just a set of lines (and the filled portion will disappear). Fills are only supported within a single object. Once you cause the object to become two or more objects, you lose the capability to fill it. You can either use Undo to restore your original object, or you can Glue all the individual line segments together.